Conventionally, catalysts containing a perovskite-type composite oxide having a crystal structure of a general formula ABO3 have been known as exhaust gas purifying catalysts for internal combustion engines.
For example, it has been reported that a perovskite-type composite oxide composed of La1.00Fe0.57CO0.38Pd0.05O3, in which lanthanum (La) is coordinated on the A site, and cobalt (Co) and palladium (Pd) are coordinated on the B site in the general formula ABO3, reversibly introduces or extracts Pd to or from a perovskite-type crystal structure corresponding to oxidation-reduction change of emissions, and suppresses grain growth and maintains high catalytic activity over a long period of time because of such a self-regenerative function (see, for example, the following Non-Patent Document 1).
However, in order to achieve the self-regeneration function of Pd of the perovskite-type composite oxide of a general formula AB(1−y)PdyO3, in which Pd is coordinated on the B site, it is necessary to form a solid solution of Pd at a high rate in the perovskite-type crystal structure. Therefore, for example, a perovskite-type composite oxide represented by the following general formula (20), in which the atomic ratio of the element on the A site exceeds 1, has been proposed (see, for example, the following Patent Document 1).AxB(1−y)PdyO3+δ  (20)(wherein A represents at least one element selected from rare earth elements and alkaline earth metals; B represents at least one element selected from transition elements (excluding rare earth elements and Pd), Al and Si; x represents an atomic ratio satisfying the following condition: 1<x; y represents an atomic ratio satisfying the following condition: 0<y≦0.5; and δ represents an oxygen excess.Non-Patent Document 1: Y. Nishihata et al., Nature, Vol. 418, No. 6894, pp. 164-167, 11 Jul. 2002Patent Document 1: International Patent Publication No. WO 2005/090238